a) Field of the Invention
The invention is directed to a method and an arrangement in fluorescence microscopy, particularly laser scanning microscopy, fluorescence correlation spectroscopy, and nearfield scanning microscopy, for examination of predominantly biological specimens, preparations and associated components. This includes methods for screening active ingredients based on fluorescence detection (high throughput screening). The transition from the detection of a few broad-spectrum dye bands to the simultaneous acquisition of whole spectra opens up new possibilities for the identification, separation and allocation of mostly analytic or functional specimen characteristics to spatial partial structures or dynamic processes. Therefore, simultaneous examination of specimens with multiple fluorophores with overlapping fluorescence spectra are even possible in three-dimensional or spatial structures of thick specimens. The spectral and spatial resolution of the detection unit is increased by means of the arrangement.
b) Description of the Related Art
A typical area of application of light microscopy for examining biological preparations is fluorescence microscopy (Pawley, “Handbook of Biological Confocal Microscopy”; Plenum Press 1995). In this case, determined dyes are used for specific labeling of cell parts.
The irradiated photons having a determined energy excite the dye molecules, through the absorption of a photon, from the ground state to an excited state. This excitation is usually referred to as one-photon or single-photon absorption (FIG. 1a). The dye molecules excited in this way can return to the ground state in various ways. In fluorescence microscopy, the most important is the transition with emission of a fluorescence photon. Because of the Stokes shift, there is generally a red shift in the wavelength of the emitted photon in comparison to the excitation radiation; that is, it has a greater wavelength. Stokes shift makes it possible to separate the fluorescence radiation from the excitation radiation.
The fluorescent light is split off from the excitation radiation by suitable dichroic beam splitters in combination with blocking filters and is observed separately. This makes it possible to show individual cell parts that are dyed with different dyes. In principle, however, several parts of a preparation can also be dyed simultaneously with different dyes which bind in a specific manner (multiple fluorescence). Special dichroic beam splitters are used again to distinguish the fluorescence signals emitted by the individual dyes.
In addition to excitation of dye molecules with a high-energy photon (single-photon absorption), excitation with a plurality of low-energy photons is also possible (FIG. 1b). The sum of energies of the single photons corresponds approximately to a multiple of the high-energy photon. This type of excitation of dyes is known as multiphoton absorption (Corle, Kino, “Confocal Scanning, Optical Microscopy and Related Imaging Systems”; Academic Press 1996). However, the dye emission is not influenced by this type of excitation, i.e., the emission spectrum undergoes a negative Stokes shift in multiphoton absorption; that is, it has a smaller wavelength compared to the excitation radiation. The separation of the excitation radiation from the emission radiation is carried out in the same way as in single-photon excitation.
The prior art will be explained more fully in the following by way of example with reference to a confocal laser scanning microscope (LSM) (FIG. 2).
An LSM is essentially composed of four modules: light source, scan module, detection unit and microscope. These modules are described more fully in the following. In addition, reference is had to DE19702753A1.
Lasers with different wavelengths are used in an LSM for specific excitation of different dyes in a preparation. The choice of excitation wavelengths is governed by the absorption characteristics of the dyes to be examined. The excitation radiation is generated in the light source module. Various lasers (argon, argon/krypton, Ti:Sa lasers) are used for this purpose. Further, the selection of wavelengths and the adjustment of the intensity of the required excitation wavelength is carried out in the light source module, e.g., using an acousto-optic crystal. The laser radiation subsequently reaches the scan module via a fiber or a suitable mirror arrangement.
The laser (Laser A-D) radiation generated in the light source is focussed in the preparation in a diffraction-limited manner by means of the objective (2) via the scanner, scanning optics and tube lens. The focus scans the specimen in a point raster in x-y direction. The pixel dwell times when scanning over the specimen are mostly in the range of less than one microsecond to several seconds.
In confocal detection (descanned detection) of fluorescent light, the light emitted from the focal plane (specimen) and from the planes located above and below the latter reaches a dichroic beam splitter (MDB) via the scanner. This dichroic beam splitter separates the fluorescent light from the excitation light. The fluorescent light is subsequently focused on a diaphragm (confocal diaphragm/pinhole) located precisely in a plane conjugate to the focal plane. In this way, fluorescent light components outside of the focus are suppressed. The optical resolution of the microscope can be adjusted by varying the size of the diaphragm. Another dichroic blocking filter (EF 1-5) which again suppresses the excitation radiation is located behind the diaphragm. After passing the blocking filter, the fluorescent light is measured by means of a point detector (PMT 1-5).
When using multiphoton absorption, the excitation of the dye fluorescence is carried out in a small volume at which the excitation intensity is particularly high. This area is only negligibly larger than the detected area when using a confocal arrangement. Accordingly, a confocal diaphragm can be dispensed with and detection can be carried out directly following the objective (non-descanned detection).
In another arrangement for detecting a dye fluorescence excited by multiphoton absorption, descanned detection is carried out again, but this time the pupil of the objective is imaged in the detection unit (nonconfocal descanned detection).
From a three-dimensionally illuminated image, only the plane (optical section or slice) located in the focal plane of the objective is reproduced by the two detection arrangements in connection with corresponding single-photon absorption or multiphoton absorption. By recording or plotting a plurality of optical slices in the x-y plane at different depths z of the specimen, a three-dimensional image of the specimen can be generated subsequently in computer-assisted manner.
Accordingly, the LSM is suitable for examination of thick preparations. The excitation wavelengths are determined by the utilized dye with its specific absorption characteristics. Dichroic filters adapted to the emission characteristics of the dye ensure that only the fluorescent light emitted by the respective dye will be measured by the point detector.
Currently, in biomedical applications, a number of different cell regions are labeled simultaneously by different dyes (multifluorescence). In the prior art, the individual dyes can be detected separately based on different absorption characteristics or emission characteristics (spectra) (FIG. 3a). For example, emission signals are plotted over wavelength for different dyes (1-4). For separate detection, an additional splitting of the fluorescent light of a plurality of dyes is carried out with the secondary beam splitters (DBS) and a separate detection of the individual dye emissions is carried out in separate point detectors (PMT x). With the arrangement described above, it is impossible for the user to flexibly adapt detection and excitation to corresponding new dye characteristics. Instead, new dichroic beam splitters and blocking filters must be created for every (new) dye. In an arrangement according to WO9507447, the fluorescent light is split spectrally by means of a prism. The method differs from the above-described arrangement with dichroic filters only in that the characteristic of the utilized filter is adjustable. However, it is still preferable to record the emission band of a dye by point detector.
Fast local measurement of the emission spectrum is possible only conditionally with the two arrangements, since the adjustment of the emission range relies on mechanical movements of the dichroic filter and diaphragms and the maximum spectral resolution is therefore limited to some 5 nm. A high spectral resolution is needed, for example, when the emission spectra overlap as is shown in FIG. 3b. FIG. 3b shows behavior of this kind in the two naturally occurring dyes CFP and GFP. These dyes are particularly suited to examination of living preparations because they have no toxic effect on the specimens to be examined.
When the position of the emission spectrum of the utilized dyes is unknown or when a shift occurs in the emission spectrum depending on environment (FIG. 3c), high-resolution detection of the dye fluorescence is necessary. The wavelength shift can amount to several times 10 nm. Spectrometers are also currently used in combination with an LSM to measure the emission spectrum in the specimen. In so doing, a conventional, usually high-resolution spectrometer is used instead of a point detector (Patent: Dixon, et al. U.S. Pat. No. 5,192,980). However, these spectrometers can record an emission spectrum only point by point or as an average over a region. Thus, this is a type of spectroscopy. In addition, the usually weak fluorescence signal of the specimen is distributed to a large quantity of individual channels in the spectrometer (usually 512 or 1024 individual channels) or a narrow fluorescence band is detected corresponding to the spectral resolution. Therefore, the signal per individual channel is extremely small and in some cases is not detectable.
Line scanners, as they are called, are also used according to the prior art in place of point scanners (Corle, Kino, “Confocal Scanning Optical Microscopy and Related Imaging Systems”; Academic Press 1996). The basic construction essentially corresponds to that of an LSM according to FIG. 2. However, instead of a point focus, a line is imaged in the focus and the specimen to be examined is scanned in only one direction. A slit diaphragm is used as confocal diaphragm instead of a pinhole diaphragm in a construction of this kind. Non-descanned detection can also be carried out with this arrangement using multiphoton absorption. The confocal diaphragm can again be omitted in this case. Usually, a CCD line with 1024 or more image points is used for detection. The image acquisition rate is increased considerably by scanning a line instead of a point. This scanning method can accordingly be used for observation of fast-running processes in real time.
It is disadvantageous in the methods according to the prior art that the line detector must usually be read out sequentially, so that increased readout noise can result at fast readout rates.